There are a wide variety of metal oxides and such metal oxides are used for various applications. Indium oxide is a well-known material and has been used as a material of transparent electrodes needed in liquid crystal display devices or the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such a metal oxide is already known (e.g., see Patent Documents 1 to 4, Non-Patent Document 1, and the like).
As metal oxides, not only single-component oxides but also multi-component oxides are known. For example, InGaO3(ZnO)m (m: natural number) having a homologous series is known as a multi-component oxide semiconductor including In, Ga, and Zn (e.g., see Non-Patent Documents 2 to 4 and the like).
Furthermore, it is confirmed that an oxide semiconductor including such an In—Ga—Zn—O-based oxide is applicable to a channel formation region of a thin film transistor (e.g., see Patent Document 5, Non-Patent Documents 5 and 6, and the like).
In order to achieve high-speed operation or the like of a transistor, the transistor needs to be miniaturized. For example, Patent Document 6 discloses a thin film transistor in which the thickness of a channel layer including an oxide semiconductor is approximately less than 10 nm, and Non-Patent Document 7 discloses a thin film transistor in which the length of a channel including an oxide semiconductor is 2 μm to 100 μm.